The invention pertains to a method for producing hot-rolled strip and plates in a production plant consisting of a continuous-casting installation for slabs with a thickness of 100-180 mm and an exit temperature from the continuous casting installation of more than 1,000xc2x0 C., a heating furnace, and a Steckel mill.
In a production plant known under the name xe2x80x9cFFMxe2x80x9d (Flexible Flat Mill) for the production of both hot-rolled strip and plates, a slab with a thickness of 100-180 mm is transported directly from the continuous casting machine over a roll table to the heating furnace, loaded while hot into the furnace, heated, and after leaving the heating furnace rolled into strip or into one or more plates in a one-stand or multi-stand Steckel mill.
The temperature of the slab after leaving the continuous casting machine is usually between 1,000xc2x0 C. and 1,150xc2x0 C. and decreases as it is being transported to the heating furnace on the roll table. The direct, hot loading into the heating furnace occurs at temperatures of 750-950xc2x0 C. In the heating furnace, the slab is heated uniformly over its thickness, width, and length to a temperature of 1,050-1,280xc2x0 C., depending on the material.
Characteristic of the hot loading technique is that, before the first deformation across the thickness of the slab on the rolling line, little or no austenite-ferrite/pearlite transformation occurs in the surface region if the surface temperatures do not fall below or fall only slightly below the transformation temperatures as the slab is being transported from the continuous casting machine to the heating furnace. The coarse-grained primary austenite which forms during solidification of the slab remains preserved for the most part until deformation on the rolling line. The size of the austenite grain can become even larger in the heating furnace, depending on the type of material in question and on the heating technology used.
In comparison to cold loading, the hot loading technique offers savings in both heating energy and time during the heating process
The technique of hot loading described above has been found reliable for steels with a copper content of less than 0.3%. At higher copper contents in the steel, the copper which is freed during scale formation in the heating furnace accumulates at the grain boundaries of the primary austenite. As a function of the copper content, the heating temperature, and scale formation, these copper accumulations at the grain boundaries can lead to material separations in the form of alligator cracks during deformation in the rolling mill.
To solve this problem, which also occurs in thin-slab casting and rolling mills, EP 0,686,702 A1 proposes that the surface temperature of 40-70 mm-thick slab be lowered to a point below the Ar3 temperature in a cooling interval following the continuous casting machine, so that, in the surface region down who a depth of at least 2 mm, at least 70% of the austenite microstructure becomes transformed into ferrite/pearlite with reorientation of the austenite grain boundaries after reheating in the roller-hearth furnace. The average surface temperature should not fall below the martensite threshold of the starting stock during cooling in the cooling interval.
It is to be observed in general that, according to the state of the art for the rolling of ingots, billets, and slabs of a certain chemical composition, cracks or material separations occur when the technique of hot loading into the heating furnace is used as a direct coupling between the continuous casting machine and the rolling mill.
In JP 59[1984]-189,001, the rapid cooling of the skin layer in the area between the continuous casting machine and the heating furnace is proposed for billets of carbon steels with 5100 ppm of boron, 0.03-0.15% of sulfur, and 0.5-2.0% of silicon in order to prevent cracks in the stock during rolling.
In EP 0,587,150 A1, AlN segregations during hot loading are held responsible for cracks in the stock during the rolling of aluminum killed steels with 0.008-0.030% of N and 0.03-0.25% of Pb. It is recommended that, to suppress the AlN segregations, the skin layer of the blooms be cooled rapidly with microstructural transformation in the bainite region. The rapid cooling takes place between the continuous casting machine and the heating furnace.
In U.S. Pat. No. 5,634,512, segregations of Al, V, and N during hot loading are given as the cause of cracks in blooms, billets, and slabs as a result of the tensile stresses which develop during air cooling. It is proposed here, too, that the skin layer be cooled rapidly to a depth of at least 10 mm to a temperature of 400xc2x0 C., followed by a self-temper to 900xc2x0 C. by the residual heat flowing from the core. The device for rapid cooling is located between the continuous casting machine and the heating furnace. A material-specific control mechanism and closed-loop control is provided for the cooling device.